Shape memory alloys (SMAs) are the group of metallic materials that exhibit a shape-memory effect (SME). The shape-memory effect is the ability to return to a previously defined shape through an appropriate thermal procedure after being severely deformed, and is a consequence of a crystallographic reversible, thermoelastic martensitic transformation (TMT). In martensitic transformations, there exists a parent, high-temperature austenite crystallographic phase having higher symmetry. As the temperature lowers, the crystallographic structure changes to martensite, a phase with lower symmetry. It is possible for multiple variants or orientations of the martensite phase to be present in the same material.
Phase transformations between austenite and martensite exhibit a hysteresis and can be activated by either temperature or stress. FIG. 1 and FIG. 2 schematically illustrate the forward and reverse phase transformations in the strain-temperature (ε-T) plane and in the stress-strain (σ-ε) plane, respectively.
Specifically, transformation through a thermocycle is illustrated in FIG. 1, where Ms (martensite start temperature) is the temperature at which martensite formation begins, Mf (martensite finish temperature) is that at which martensite formation terminates, As (austenite start temperature) is that at which the reverse transformation begins, and Af (austenite finish temperature) is that at which the reverse transformation terminates. When a SMA is heated above As, the martensite variants change to reproduce the shape of the parent phase.
FIG. 2 corresponds to the case when an exemplary SMA is deformed at a constant temperature. This temperature is preferably in the range between Af and Md where Af is the previously defined austenite finish temperature, and Md marks the maximum temperature at which martensite can be induced by an applied stress without substantially plastically deforming the austenite phase. With an increase in stress level (analogous to a decrease in temperature in the previous case), the martensite phase starts to form in the material. The line from point A to point B represents the elastic deformation of a SMA. After point B, the strain or deformation is no longer proportional to the applied stress, and it is in the region between point B and point C that the stress-induced transformation of the austenitic phase to the martensitic phase begins to occur. There also can be an intermediate phase, called the rhombohedral phase (or more commonly, the “R-Phase”), depending upon the composition and the thermomechanical history of the alloy. At point C moving toward point D, the material enters a region of relatively constant stress with significant deformation or strain. This constant stress region is known as the loading plateau, and it is in this plateau region C-D that the transformation from austenite to martensite occurs.
At point D the transformation to the martensitic phase due to the application of stress to the specimen is substantially complete. Beyond point D the martensitic phase begins to deform, elastically at first, but, beyond point E, the deformation is plastic.
When the stress applied to the superelastic metal is removed, the material behavior follows the curve from point E to point F. Within the E to F region, elastic recovery occurs. At point F in the recovery process, the metal begins to transform from the metastable, martensitic phase back to the more stable austenitic phase.
In the region from point G to point H, which is also an essentially constant or plateau stress region, the phase transformation from martensite back to austenite takes place. This constant stress region G-His known as the unloading plateau. The line from point I to the starting point A represents the elastic recovery of the metal to its original shape.
The ability to incur significant strain under an applied stress and to recover from the deformation upon removal of the load is commonly referred to as “superelasticity” and sometimes “pseudoelasticity.” In an ideal system, superelastic behavior is characterized by regions of nearly constant stress upon loading and unloading, identified above as loading plateau stress C-D and unloading plateau stress G-H.
Numerous alloys having shape memory effect and superelasticity are known, such as silver-cadmium (Ag—Cd), gold-cadmium (Au—Cd), gold-copper-zinc (Au—Cu—Zn), copper-aluminum-nickel (Cu—Al—Ni), copper-gold-zinc (Cu—Au—Zn), copper-zinc (Cu—Zn), copper-zinc-aluminum (Cu—Zn—Al), copper-zinc-tin (Cu—Zn—Sn), copper-zinc-xenon (Cu—Zn—Xe), iron beryllium (Fe3Be), iron platinum (Fe3Pt), indium-thallium (In—Tl), iron-manganese (Fe—Mn), nickel-aluminum (Ni—Al), copper based alloys, as well as Ni—Ti (nickel-titanium) based alloys. Moreover, equiatomic or near-equiatomic nickel-titanium alloys (i.e., Nitinol) are very important for practical uses owing to their physical, mechanical and chemical properties, performance specifications, corrosion resistance and SME. In particular, Ti—Ni alloys can exhibit 50-60% elongation and have a tensile strength up to 1000 MPa. Upon heat treatment, Ti—Ni transforms from a ductile martensite phase with B19′ structure to a stiffer austenite phase of B2 structure. When cooled, Ti—Ni shifts in structure from B2 to B19′. In bulk equiatomic Ti—Ni, Ms is approximately within the range of 60° C.-65° C., and As is approximately 95° C.-100° C. These transformation temperatures, however, can be significantly lower for thin films and are highly dependent on composition and residual stresses. The martensitic transformations depend on the chemical composition of the alloy. Thus, when Nitinol is alloyed with other metals, cooling of the material can provide transformation of B2 structure to either B19 structure or to the so-called R-phase, and then to the B19′ structure.
SMAs may be practically used for actuators, pipe couplings, switches or the like by taking advantages of their shape memory properties. Products that rely on the superelasticity of the SMAs include, but are not limited to, antennas, eye glass frames, wires of brassieres, orthodontic archwires, etc. Various proposals have also been made to employ SMAs in the medical field. For example, U.S. Pat. No. 3,620,212 to Fannon et al. proposes the use of an SMA intrauterine contraceptive device, U.S. Pat. No. 3,786,806 to Johnson et al. proposes the use of an SMA bone plate, U.S. Pat. No. 3,890,977 to Wilson proposes the use of an SMA element to bend a catheter or cannula, etc. In this connection, the functional capabilities of nickel-titanium alloys are especially important in medicine because their biochemical properties best match the mechanical behavior and properties of human living tissues. Various nickel-titanium alloys can also be used in guide wires, cardiac pacing leads, sutures, prosthetic implants, such as stents, intraluminal filters, retrieval baskets, aneurysm clips, bone plates and screws, femoral fixation devices, intramedullary nails and pins, and joints for ankles, elbows, fingers, knees, hips, shoulders and wrists. Such nickel-titanium alloys are described in, for example, U.S. Pat. Appl. Pub. Nos. 2001/0001317 to Duerig et al., 2004/0249447 and 2006/0086440 to Boylan et al., 2004/0143317 to Stinson, 2004/0236409 to Pelton et al., 2004/220608 to Wayne et al., 2005/209683 to YAMAUCHI, U.S. Pat. No. 5,951,793 to Mitose et al., U.S. Pat. No. 6,312,455 to Duerig et al., U.S. Pat. No. 6,306,141 to Jervis, U.S. Pat. No. 6,676,700 to Jacobs et al., U.S. Pat. No. 6,926,733 to Stinson, U.S. Pat. No. 6,776,795 to Pelton et al., U.S. Pat. No. 6,855,161 to Boylan et al.).
Although nickel-titanium alloys are useful and valuable to the medical field, one of the disadvantages of the medical implants made of the known SMAs is the fact that they are not sufficiently radiopaque, as compared to, for example, devices made of medical stainless steel.
The radiopacity of medical implants can be improved in several ways. For example, virtually regardless of the energy and wavelength of incident x-rays, material having a high mass per unit of illuminated area (material density) is radiopaque; indeed, the radiopacity of high mass constructions is generally higher than that of low mass constructions. However, the extensive use in medical practice of small-size articles made from materials having SME and superelasticity imposes limits on the utilization of high mass constructions.
Radiopacity can also be improved through coating processes such as sputtering, plating, or co-drawing high radiopacity metals onto the medical device. These processes, however, may create complications such as material compatibility, galvanic corrosion, coating adhesion or delamination, biocompatibility, etc.
Radiopacity can also be improved by alloy addition. A challenge for medical device applications is the preparation of a suitably radiopaque Ti—Ni alloy that also displays superelastic behavior around body temperature. It would be advantageous to develop a radiopaque nickel-titanium alloy that provides a substantial improvement in radiopacity compared to binary Ti—Ni alloys without a loss in superelastic properties.